CAUSES OF OPERATIONAL FATIGUE IN FRAME STRUCTURES AND MEASURES FOR ITS PREVENTION

ABSTRACT

In this paper, catastrophic failure of large-size metal structures is a serious problem for engineering structures. The results of the study show that traditional calculation methods cannot adequately assess the influence of defects. Defects in metal structures occur as a result of manufacturing, technological processes and operation. These factors lead to the formation of cracks and reduce the strength of the structure. The study developed recommendations to improve the reliability of metal structures based on non-destructive testing methods, diagnostic technologies and load analysis. This approach helps to extend service life and prevent accidents.

АННОТАЦИЯ

В данной статье катастрофическое разрушение крупногабаритных металлических конструкций является серьёзной проблемой для инженерных сооружений. Результаты исследования показывают, что традиционные методы расчета не могут адекватно оценить влияние дефектов. Дефекты в металлических конструкциях возникают в результате производства, технологических процессов и эксплуатации. Эти факторы приводят к образованию трещин и снижению прочности конструкции. В ходе исследования были разработаны рекомендации по повышению надежности металлических конструкций на основе методов неразрушающего контроля, диагностических технологий и анализа нагрузок. Этот подход помогает продлить срок службы и предотвратить аварии.

Keywords: Metal structures, catastrophic destruction, destruction mechanics, fatigue cracks, operational loads, welded joints, fatigue process, diagnostic methods, strength and reliability.

Ключевые слова: Металлические конструкции, катастрофическое разрушение, механика разрушения, усталостные трещины, эксплуатационные нагрузки, сварные соединения, процесс усталости, методы диагностики, прочность и надежность.

INTRODUCTION.

The catastrophic failure of large-scale metal structures is one of the serious problems in the fields of mechanical engineering, construction, and operation of complex engineering facilities. Analysis of actual failure cases shows that traditional calculation methods based on classical approaches in mechanics do not always allow for adequate prediction of the probability of such accidents. One of the reasons for this is that computational models often assume ideal homogeneity of the material, while in reality, metal structures contain various defects that arise during the stages of production and operation [1-4]. These defects in the frame structure are classified according to their origin as follows:

• Defects in metal production - flaws associated with the processes of melting, casting, and rolling of metal;
• Technological defects - shortcomings in the processes of machining, welding, heat treatment, and assembly of the structure;
• Operational defects are damages that occur due to external loads, fatigue processes, and the effects of corrosion.

Under certain conditions, these defects can become a source of localized stresses, which leads to the formation of cracks, a decrease in the structural strength, and ultimately, its brittle failure. Thus, to increase the reliability of metal structures, it is necessary to consider not only the maximum allowable stresses but also factors associated with material defects, its microstructure, and fatigue processes. The rate of crack initiation and subsequent propagation in metal structures depends on many factors, among which the main role is played by the internal structure of the material, structural parameters of the frame, nature of external loads, as well as operating conditions such as aggressive environments and temperature regimes.

During operation, static, dynamic, cyclic, and impact loads act on the structure, causing the accumulation of damage in the material over time. This is especially relevant for elements operating under variable loads, where fatigue damage mechanisms play a significant role. In recent years, the principles of material failure for simple loading schemes have been studied in detail. In actual operating conditions, it is crucial to consider all possible factors influencing failure: microstructural properties of the material, presence of residual stresses, temperature fluctuations, aggressive environments, mechanical fatigue, and dynamic overloads [5-6].

RESULTS AND ANALYSIS OF RESULTS

Despite the achievements in fundamental research of fracture mechanics, its application in engineering remains limited. In particular, there are insufficient general studies devoted to the strength and reliability characteristics of railway rolling stock from the perspective of materials fracture mechanics. The implementation of fracture mechanics methods in various industries, including transport engineering and construction, is an important task, allowing to increase the safety and durability of operating facilities. In the field of engineering, particularly in ensuring the safety and durability of load-bearing elements of locomotives, a comprehensive approach to studying the materials of metal structures and their welded joints is necessary.

In the field of engineering, especially in ensuring the safety and durability of load-bearing elements (main frame and bogie frame) of locomotives, a comprehensive approach to studying the properties of metal structure materials and their welded joints is of great importance. This approach provides for a thorough analysis of structural materials' behavior under various loading modes - static, dynamic, and cyclic. Special attention is paid to the influence of operational factors, such as aggressive environments, temperature fluctuations, and random loads, which allows for a more accurate prediction of the service life of metal structures [7]. Currently, as part of the process of extending the service life of electric locomotive VL60 No. 2070, whose service period has expired according to the regulatory and technical documentation of the JSC “O‘zbekiston temir yo‘llari” locomotive fleet, major repair work is being carried out at the JSC “O‘ztemir yo‘lmashta’mir” enterprise. The work on extending the service period has been carried out in accordance with the requirements of interstate standards.

Figure 1. VL60 No. 2070 electric locomotive that has reached the end of its service life

One of the most important aspects of the research is determining the limit state parameters of metal structures subjected to fatigue damage. Studying the processes of fatigue damage accumulation, crack formation, and propagation allows for identifying hazardous zones in the structure and establishing permissible load limits. This data is essential for developing effective diagnostic methods and timely prevention of emergency malfunctions [8]. Within the framework of a comprehensive analysis, a method for calculating the durability of metal structures, as well as assessing the residual life of their cracked elements, is presented.

In addition to the methodology for calculating safe and critical parameters of fatigue cracks, methods for local and general technical diagnostics have been developed. Local methods, such as capillary, magnetic particle, and ultrasonic testing, enable the detection of surface and internal defects in the early stages of their development [9-10]. Concurrently, general diagnostic methods based on vibration analysis, acoustic emission, and deformation monitoring allow for assessing the overall condition of the structure and predicting potential failure points. Below is an example of metal structure failure due to fatigue during the overhaul of a VL60 electric locomotive in the locomotive fleet of JSC “O‘zbekiston temir yo‘llari” which had reached the end of its service life.

Figure 2. The condition of a broken bracket component of a VL60 electric locomotive with expired service life due to long-term fatigue; a) general view of the bracket, b) and c) the broken state of the bracket

To enhance the durability of frame structures in terms of materials and welding, it is necessary to optimize metallurgical processes and welding technologies. This will serve to increase the reliability of metal structures, extend their service life, and prevent accidental failures.

CONCLUSION

Catastrophic failures of metal structures are associated with defects arising from production, operation, and external influences. Studies show that traditional calculation methods are insufficient for a comprehensive assessment of these processes. It is crucial to identify the initial stages of cracks and extend the service life through fracture mechanics and non-destructive testing technologies. Studying fatigue damage in railway locomotives and developing diagnostic methods help increase the reliability of metal structures. Among the recommendations, analyzing operational loads and reinforcing welded joints play an important role.

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